1. Field of the Invention
This invention relates to field-effect transistors in general and, more particularly, to multiple quantum well heterojunction field-effect transistors.
2. Description of the Prior Art
Large signal amplifiers, such as power amplifiers, require careful design to minimize nonlinear distortions, such as harmonic or intermodulation distortions, of the signals being amplified. For example, in power amplifier designs used for radio frequency transmitters, low-pass filter networks are provided at the output of the power amplifier to reduce the harmonic content of the output signal below Federal Communication Commission mandated harmonic and spurious signal levels (typically expressed in decibels below the output signal power level). The higher the harmonic content of the output signal from the power amplifier, the more complex the filter arrangement and the less efficient the power amplifier becomes. Also, many signal modulation formats used for communication systems are intolerant to high levels of in-band intermodulation distortion caused by amplifier nonlinearities which cannot be removed with filters. As a result, the power amplifier operates at a output power level significantly below the maximum output power level of which it is otherwise capable of in order to reduce intermodulation distortion to acceptable levels. Such intermodulation distortion requirements can necessitate the use of a larger, costlier, amplifier than would be required solely on the basic output power level considerations. To reduce the harmonic distortion, the power amplifier may be designed as a class A, B, AB.sub.1, or AB.sub.2 amplifier which, through circuit techniques and biasing arrangements, reduce the harmonic distortion to less than is possible from a class C, D, or S amplifier. However, the output signal harmonic and intermodulation distortion content of a class A, B, AB.sub.1, or AB.sub.2 amplifier may still be higher than that desired. A major cause of harmonic and intermodulation distortion in these classes of amplifiers is the non-linear transfer characteristics of the active device or devices used in the amplifier.
Another application where low amplifier distortion is especially desirable is in a receiver preamplifier. The performance of a receiver may be significantly degraded if the first stage amplifier thereof distorts strong signals entering the receiver. The dominant effect of distorting the entering signals, particularly those signals having a frequency near the frequency of the desired signal and having a much larger amplitude, is the cross-modulation or intermodulation of the desired signal by the "unwanted" signal(s). The cross-modulation can distort the desired signal to the point of making it unusable. Thus, the preamplifier is usually a class A amplifier, typically the most distortion-free of all amplifier classes. However, as mentioned above, the minimum possible distortion level is set by the non-linearity of the active device(s) in the first stage amplifier.
Substantially all conventional active devices, such as bipolar transistors or field-effect transistors, have non-linear transfer characteristics, i.e., the AC gain (transconductance) of the device changes with the amplitude of the input signal. For example, the transfer characteristic of a field-effect transistor (FET) follows a "power" law (usually either square or 3/2 power) instead of a "linear" law (a "linear law" being indicative of constant transconductance over a large range of input power levels). A "power" law transfer characteristic, under large signal conditions, causes nonlinear distortions in the amplified signal.
Another advantage of "linear" law (nearly constant transconductance) over "power" law transfer characteristics is the obtaining of higher dc to rf and power-added efficiencies in high-efficiency power amplifier circuits which are typically designed to work in Classes other than Class A (e.g., Class B or Class C). A "linear" law (constant or nearly constant transconductance) device exhibits less power gain degradation and higher efficiencies than "power" law devices because of the "linear" nature of its transfer characteristics.